Method of lubricating



Feb. 12, 1963 w. c. CLINTON ErAL 3,077,242

METHOD 0F LUBRICATLNG Filed Deo. 2, 1960 40T EvAPoRATEn FILMS or cAPRYLlc Acm FIG 2 EvAPoRATEn F oF u, cAPRYLlc 22- ro 20-- f f T i l' l 'T E! E a T n: n: l a D N- j RoToR OVW 1 2" J waz 1.0 i a i TEMPERATURE INVEN RS WILLIAM C. CLINTON i I ERE E. B HY 1000 127 150 TA CHENG TEMPERATURE IN F ATTORNEY Unite States Patent ffice 3,077,242 METHOD F LUBRICATING William C. Clinton, Binghamton, NY., 1ere E. Brophy, Cambridge, Mass., and Ta Cheng Ku, Endicott, N.`Y., nssignors to International Business Machines Corporation, New York, NY., a corporation of New York Filed Dec. 2, 1960, Ser. No. 73,353 2 Claims. (Cl. 3,84-1) This invention relates to a method of lubricating moving surfaces, and more particularly to a method of providing dynamic lubrication by vaporization techniques.

It is a principal object of this invention to provide a method of lubricating by vaporization techniques surfaces which move relative to one another such as bearings.

It is a more specific object of this invention to provide a method of lubricating the surfaces of electrostatic clutches.

Although the following description is directed to a method for lubricating the sliding surfaces of an electrostatic clutch, it will be understood at the outset that the method, according to the invention, is generally useful in lubricating two relatively sliding surfaces or bearings.

Electrostatic clutches comprise a driving member clutched to a driver member; one member is of a conductive material and the other member is of a semiconductive material. The electroadhesive effect between conductors and semiconductors, as utilized in electrostatic clutches, is well known in the art, and there are a number of materials that will exhibit this effect.

A primary advantage of an electrostatic clutch over other similar devices is rapid electrical response time. In addition to rapid response time, an electrostatic clutch has many other potential advantages not found in similar devices such as, for example, a low operating current in the order of a milliampere at 200 volts, noninductive circuit operation, low manufacturing costs, and small size in relation to the torque output.

Heretofore, it has been found that due to normal welding and adhesion of two clean sliding surfaces of the clutch, transfer of the semi-insulating material to the surface of the metallic band occurs as the assembly is rotated. Accompanying this transfer is a constant decrease of the force of electrostatic attraction, which force approaches zero with time as the transfer increase. It follows that the usefulness of such a clutch is limited unless means are employed to reduce the material transfer and to maintain a constant force of electrostatic attraction.

Further, dust particles which may be deposited on a clutching surface cause amplified wear on the clutching surfaces, since the dust particles act as abrasives between the surfaces. The amplified wear on the clutch results in alterations of the quality of the clutching surfaces, that is, in the smoothness of the clutch surface and produces unstable and unreliable operation. Thus, lubricants are needed on the clutching surface to reduce wear between the surfaces.

It is another object of this invention to provide a method of adsorbiug a thin film on a sliding interface to prevent material transfer therebetween.

It is yet another object of this invention to provide a method for adsorbing and maintaining on a sliding interface a lubricating film having the optimum molecular dimensions necessary to produce a mechanically useful tension in the metallic band.

It is still another object of this invention to provide a method for controlling the effect of the adsorbed film on the frictional characteristics and the tension developed in the metallic band of an electrostatic clutch.

In one preferred embodiment, the invention provides a method of forming a thin film between a driving member and a driven member by positioning an oven adjacent the interface of said members, placing the lubricating material in the oven, heating and vaporizing the material, and maintaining a fixed temperature differential between the oven and the interface of said members to cause a definite thickness of the material to be adsorbed .and maintained on the interface.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 shows a schematic diagram of a structure utilized in practicing the method according to the invention;

FIGS. 2 and 3 show graphs useful in explaining the operation of the method according to the invention.

Referring to FIG. l, an oven 11 comprises a cavity 13 in which is placed a chemical compound, a mixture of chemical compounds, or any lubricating media 14. Oven il is positioned adjacent a rotor 25 of the electrostatic clutch 20. Rotor 25 is of a semiconductive material. A metallic band 27 is actuated or driven by the rotor 25. Band 27 is held under an initial tension by a spring 39 connected to its one end; the other end of the band may be connected to any suitable utilization device. Oven 11 includes a heater 30 for heating and vaporizing the compound 14, and a passageway 23 for guiding the vaporized material onto the rotor 25. A thermocouple 29 is mounted in oven 11 for determining the temperature thereof. A second thermocouple 31 measures the approximate temperature of the contacting surfaces of the rotor 25 and band 27 for purposes of controlling the temperature differential between the oven or, more specically, for controlling the temperature differential between the compound 14 and the rotor 25 for purposes `to be discussed hereinbelow.

The operation of the device is as follows. Initially, a few milliliters or milligrams of the lubricating compound 14 which is to be evaporated is placed in cavity 13. The rotor 2S is activated to rotate and the heater 30 is then energized to heat and evaporate the compound 14. After the oven has reached a predetermined steady state temperature, the device is ready for operation and switch 33 can be closed to connect battery 35 across the rotor 2S and band 27 to develop an electrostatic force therebetween. The vaporized material passes through passageway 23 and is adsorbed in rotor 2S which is engaging, that is, rubbing against band 27. The evaporated lm formed on rotor 25 provides a lubricating medium between the rotor 25 and the band 27. As will be discussed more fully hereinbelow, the thickness of the film formed on the rotor has been found to be a function of the temperature differential between the rotor 25 and the temperature of the oven 11.

ln one particular embodiment, films of polar paraflinic compounds were formed on the rotor 25. Other aliphatic, alicyclic, aromatic, or heterocyclic carbon compounds could be used; also, inorganic compounds capable of being evaporated might be employed. A typical temperature curve and the correlation diagram of the measured values of tension when films of caprylic acid are employed is shown in FIG. 2; specifically, FIG. 2 shows the relation of the oven and rotor temperature versus the tension developed in the band when using a film of caprylic acid adsorbed on rotor 25.

The more important parameter in the process of the invention is the surface temperature of the .rotor 25. The oven 11 functions to maintain an adequate vapor pressure over the surface of the rotor 25. Any change of the oven 11 temperature changes both the vapor pressure and the surface temperature of the rotor 25. After equilibrium has been established between the rotor 25 surface temperature and the oven 11 temperature, the temperature ofthe surface of the rotor will be less than the oven tem perature due to the cooling effect of the atmosphere (outside of the oven) on the rotating rotor surface.

When the rotor surface'temperature is increased from 108 F. to 110 F., by increasing the oven temperature from 123 F. to 127 F., the tension in the band increases from 1.2 lbs. to 2.2 lbs., FIG. 2. It can be shown by ellipsometric measurement that when the rotor temperatureA is 108 F. or less than two or more monolayers of caprylic acid are adsorbed at the rotor surface-band interface. When the rotor temperature is between 110 F. and 118 F., a single monolayer can be measured. The rotor transition temperature reflects the energy level necessary to desorb the first and second monolayers. Transition temperatures were not detected for films thicker than two monolayers apparently because the coefficient of friction remained constant for the thicker lms.

The response curve shown in FIG. 2 and discussed above is typical of the curves obtained when using films formed of the other compounds enumerated above. The actualv Values of friction and transition temperatures vary somewhat.

Thetransition temperatures were found to be dependent on the rotor surface temperature and on the chemical compound and its molecular structure. T he amount of tension developed at the transition was dependent upon the value ofthe electrical resistance of the rotor-band interface, which varies with each rotor and the particular area on the rotor surface contacted by the band. Also, the oven temperature necessary to reach a critical rotor transition temperature varied with each rotor. The transition from two monolayers to one monolayer occurs at some critical rotor` temperature irrespective of oven ternperature.

Referring now to FIG.4 3, there is shown the increased baud tension with increased rotor temperature for another evaluation of caprylic acid. At rotor temperatures above, approximately 127 F., the tension developed in the band increases to approximately 4 lbs. With time, this new force wil decrease to zero due to transfer of rotor material to the hand. Thus, life expectancy of an electrostatic clutch can be infinite when the material from the rotor surface is prevented from transferring to the band by maintaining one or more monolayers of lubricating compounds on the rotor surface-hand interface. The rotor temperature is maintained below a definite level since, at elevated temperatures, either the tension developed by the band becomes excessive or material transfer occurs. The latter case occurs whenever the rotor surface temperature reaches a critical level at which the desorption rate exceeds the adsorption rate for a given compound, that is, the films became nonunimolecular or nonexistent.

From the foregoing, it is seen that by controlling the temperature of the oven 11 and the rotor 2S, and the relative temperatures. thereof, the thickness of the film formed on the interface may be controlled.

It has been found. that within a small range, that is, within monolayer considerations, films which are very thin compared to the gap thickness and to the surface roughness of the rotor do not have any significant effect upon the values of the dielectric coefficient and of the equivalent interface gap thickness. The latter two parameters are functions of the tension developed in the band.

lt was also found that the friction developed between the rotor and the band when lauric acid is adsorbed on the rotor is less than that when caprylic acid is adsorbed thereon. The difference suggests that chain length of the adsorbed film has some effect on the amount of friction developed between the band and the rotor.

ln summary, dynamically evaporated films show a transition temperature different for each compound, above which one monolayer is maintained and below which two or more monolayers are maintained as adsorbed films on the rotor surface. A similar temperature exists for the transition of one monolayer to a nonunimolecular layer. A single monolayer can be maintained as an adsorbed film on the rotor surface at any temperature between the two transition points; the tension developed will be constant and of a predictable magnitude. At least one or more monolayers of adsorbed films must be continuously maintained on the rotor surface to prevent material transfer, and the amount of tension developed in the band is greater for a monolayer than for multilayers.

`While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.

What is claimed is:

l. A method of applying a film of caprylic acid cornpound between the interface or the rotor and the band of an electrostatic clutch comprising, the steps of, placing the lubricating compound in an oven, driving said rotor until a steady state temperature is achieved, heating the compound tovaporization at atmospheric pressure andv at a temperature in the range of from 123 F. to 127 F., channeling the vaporized compound onto said rotor surface, and maintaining said rotor surface at a temperature in the range of 108 F. to 118 F. to thereby cause a monolayer of compound to be adsorbed on said rotor surface whereby useful tension is developed between said rotor and said band, and whereby said rotor surface is lubricated while transfer of said compound between said rotor and said band is prevented.

2. A method of lubricating a rotor surface comprising the steps of heating a caprylic acid compound to vaporize said acid compound, channeling said acid compound onto said rotor surface, and maintaining said rotor surface in the range of 108 F. and 118 F. to thereby cause a monolayer of acid compound film to be adsorbed on said rotor surface. h

References Cited in the file of this patent UNITED STATES PATENTS 2,948,355 Hull Aug. 9, 1960 

1. A METHOD OF APPLYING A FILM OF CAPRYLIC ACID COMPOUND BETWEEN THE INTERFACE OR THE ROTOR AND THE BAND OF AN ELECTROSTATIC CLUTCH COMPRISING, THE STEPS OF, PLACING THE LUBRICATING COMPOUND IN AN OVEN, DRIVING SAID ROTOR UNTIL A STEADY STATE TEMPERATURE IS ACHIEVED HEATING THE COMPOUND TO VAPORIZATION AT ATMOSPHERIC PRESSURE AND AT A TEMPERATURE IN THE RANGE OF FROM 123* F. TO 127* F., CHANNELING THE VAPORIZED COMPOUND ONTO SAID ROTOR SURFACE, AND MAINTAINING SAID ROTOR SURFACE AT A TEMPERATURE IN THE RANGE OF 108*F. TO 118*F. TO THEREBY CAUSE A MONOLAYER OF COMPOUND TO BE ABSORBED ON SAID ROTOR SURFACE WHEREBY USEFUL TENSION IS DEVELOPED BETWEEN SAID ROTOR AND SAID BAND, AND WHEREBY SAID ROTOR SURFACE IS 